Existing wastewater treatment systems suffer from the problem of solid accumulation. The solids from airborne and biomass debris reduce water residence time in the wastewater treatment system, and as a result, negatively affect system performance. Solids are typically removed from wastewater through use of settling tanks or ponds that are unmixed. Specifically, the solids settled by gravity are periodically dredged for subsequent transport to landfills or for use in microbial digesters for biogas generation and then disposal of remaining solids. Removal of any remaining suspended solids is required prior to water outflow. Both operations can cost millions of dollars annually depending on the size of the operation and can require temporary shutdown of partial or all wastewater treatment operations.
Another problem caused by the reduced residence time is higher residual concentrations of nuisance nutrients such as nitrogen (N), phosphorus (P), and sulfur (S) in the final effluent. Increased levels of N and P often exceed the permitted outflow limits for these systems, resulting in potential fines and possible facility shut down for non-compliance of government regulations. Nutrient recovery in the form of biomass becomes attractive for managing these problems while creating value with the biomass.
Other problems stem from oxygen deprivation in the wastewater treatment system. Oxygen is required for the normal processes that convert organic matters to carbon dioxide and water. Additional oxygen can be introduced, via mechanical agitation using surface air, into the wastewater treatment system; however, this approach is inefficient and energy expensive because of the relatively high density of water and its attendant energy consumption.
U.S. Patent Application Publication No. 2010/0237009 A1 describes use of phototactic heterotrophic eukaryotes that can be enriched within the slipstream of wastewater being treated due to their phototactic properties.
U.S. Pat. No. 7,977,085 teaches use of a monoculture of photosynthetic eukaryotic algae, specifically a chlorophyte that is planted de novo in an aqueous environment to cultivate said algae under a continuous stream of carbon dioxide. Disadvantageously, it is difficult to culture algae of a single species over time in existing wastewater systems without continuous enrichment for the species, especially if many other competing microbes are naturally present in wastewater, and if high-rate nutrient recovery is desired.
The ability of microbe consortia to proliferate in low to high salt and low to high pH media at variable temperatures allows scalable mass cultivation, notably in open basins common to wastewater processing. In these conditions, the consortia are compatible with predators and with biologic agents added for specific purposes such as degradation of hydrocarbons from crude oil. The consortia are also compatible with intermittent adjustments of pH through use of a discontinuous stream of carbon dioxide as is currently practiced in wastewater operations. Added CO2 produced biomass yields double that of a paddlewheel-mixed wastewater pond without added CO2, to about 60 MT/ha-yr (0.3 MT/million L, 0.3 g/L), along with the associated acceleration of nutrient value reductions, and it improved bioflocculation for algal harvest (Craggs, R. J., S. Heubeck, T. J. Lundquist, J. R. Benemann. 2011. Algal biofuels from wastewater treatment high rate algal ponds. Water Science & Technology 63: 660-665).
Nutrient recovery during log phase growth is clearly influenced by biomass loading density and by baseline N and P content (Tam N. F. Y., Y. S. Wong. 1989. Wastewater nutrient removal by Chlorella pyrenoidosa and Scenedesmus sp. Environmental Pollution 58:19-34). As two examples, recovery efficiency after 3 days using photosynthetic algae was 20% N and 29% PO4, with P at inoculation densities starting with 5×105 cells/ml and 45% N and 52% PO4 and P removal at densities with 5×106 cells/ml under an open system of municipal wastewater with presence of common bacteria and protozoa (Lau P., N. Tam, Y. Wong. 1995. Effect of algal density on nutrient removal from primary settled wastewater. Environmental Pollution 89: 59-66). In other systems, seeding with Scenedesmus at 1×106 cells/ml with midday temps of 30-34 C resulted in cell doubling in 3 days, reaching 8-fold higher in 8 days, with N reduction from 23.5 mg/L to 3.1 mg/L (Andrade C E, A Vera, C Cárdenas, E. Morales. 2009. Biomass production of microalga Scenedesmus sp. with wastewater from fishery. Rev. Téc. Ing. Univ. Zulia. Vol. 32:126-134).
Pond management affects loading density. An unmixed pond of 1-meter depth may show a 0.05 g/L biomass density (0.05 MT/million liters) for an algae biomass productivity of about 10 metric tons/ha-yr. In contrast, a paddlewheel-mixed pond at shallower depths such as 0.3-meter depth may show a 0.2 g/L density or 0.2 MT/million L (Craggs, Heubeck et al. 2011).
Advantageously, to promote biologic proliferation for continued effective oxidation, the known practice of adding a hydrocarbon source to a basin during colder temperatures can simultaneously benefit recovery of N, P and other element values by photosynthetic eukaryotes and prokaryotes, as part of the consortium. This is applicable if the photosynthetic eukaryotes and prokaryotes are known mixotrophs that preferentially thrive over strict phototrophs because they can metabolize the added hydrocarbons while still photosynthesizing, albeit at a slower rate under colder temperatures. In this manner, the biomass yield per unit area increases by increasing the culture depth.
One major obstacle in the improvement of microbe-based wastewater treatment is the time and cost associated with designing and building de novo structures that are not normally part of a wastewater system to accommodate processing of a slipstream of wastewater via proliferation of algae biomass. Such an approach is described in, for example, U.S. Patent Application Publication No. 2010/0237009 A1 with the use of purpose-built bioreactors to treat the wastewater. It is not necessary to create a “new” wastewater treatment system designed around the physical constraints of growing or concentrating microbial biomass. For example, U.S. Pat. No. 6,896,804 teaches growing and continuous delivery of photosynthetic microbial cultures into existing wastewater basins, with the culture comprising an assemblage of various microalgae.
Energy company operations, such as those performing hydraulic fracturing and oil refining, are ideal for such operations, offering practical advantages and strong techno-economic rationale for coupling greenhouse gas and liquid effluents with large-scale microbe cultivation in wastewater treatment basins. The principles and designs easily extend to other municipal and industrial wastewater treatment facilities. Similar to other industrial wastewater facilities, each oil company can have its own upstream and downstream wastewater make-up, and composition can vary with geography, crude lots, and processes at a single site or refinery. Microbe consortia can adjust to these variations over time such that the nutrient recovery or other compositional modifications within the ponds is maintained. This approach further demonstrates that treatment facilities can monetize their solids to swiftly recoup the expenditures for system upgrades.
Further, there is a lack of effective and energy efficient mixing methods capable of preventing settling of solids through the basin while producing the desired growth of biomass in situ at a high yield without reducing hydraulic flow and system operations. U.S. Pat. No. 6,896,804 describes mechanical agitation to disperse introduced algae cultures grown ex situ. Disadvantageously, mechanical agitation is highly inefficient for increasing exposure of active volumes to light for photosynthesis in large basins and is highly energy intensive.
In addition, separation of solids from the liquid part of the effluent is required for subsequent processing of the biomass. Many means for separation of solids from the liquid are known in the art, such as use of floating suction dredgers and thickening drums or filters, centrifugation, or flocculation with flotation, or by use of polyelectrolytes and forced flotation using compressed air. However, flocculants may cause toxicity issues for native species exposed to the outflow and for many flocculants the microbes are no longer viable for recycling purposes.
Accordingly, a substantial need exists for a method for improved hydraulic wastewater treatment basins that will enable a process of nutrient recycle to support microbial biomass, especially biomass capable of producing desired bioenergy and other components and, preferably, with high yields and at a low cost. As will be clear from the disclosure that follows, these and other benefits are provided by the present invention.